The applicants proposed to design, construct, and test a prototype scanning device capable of reconstructing regional, optical absorption in three dimensions within the human breast. The device will consist of an array of sixteen, .25" diameter transducers aligned along a 60 degree, 15 cm radius arc. This arc of transducers will be scanned axially to acquire a set of photoacoustic "projections" over a little less than 2p steradians. Photoacoustic image reconstruction will be performed using a modified, inverse Radon transform. Imaging performance will be evaluated in terms of contrast/noise and spatial resolution as a function of irradiating energy (infrared and microwave) and depth within a tissue-mimicking phantom. Based on the Phase I results, specifications will be determined for a clinical prototype photoacoustic scanner, to be designed, constructed and tested in Phase II.